This tool, along with a pitot tube and a digital manometer, allow you to
accurately measure the airflow within a duct, such as that of a kitchen range hood
or dryer vent.
Because ASHRAE 62.2 requires the measurement of existing and installed
ventilation fans, the pitot tube is a useful device to have. Additionally,
for the sake of determining possible back drafting of vented combustion
appliances, it is important to be able to measure the flow of clothes
dryers, rather than estimating their airflow.
In order to find airflow from velocity pressure, calculations are required;
this is what this Pitot Tube Airflow tool will do for you. First, it helps
you calculate the air velocity based on pitot tube measurements and
air density. Then it calculates the
corresponding airflow in the duct by helping you determine the duct
The pitot tube was invented by Henri Pitot, a French engineer, in the
18th century. This ingenious device solved the problem of not being able to directly
measure the velocity pressure of airflow. This is accomplished by subtracting
the measured static pressure from the measured total pressure to find velocity
(dynamic) pressure [total pressure - static pressure = velocity pressure].
The device is very useful for measuring airflow in a dryer duct or the duct
of a ventilation fan.
The static or bursting pressure in a duct "pushes" against the walls of the
duct, similar to what happens to the wall of a balloon when you blow it up.
The velocity pressure is created by the speed of the airflow and its density.
This is similar to what happens when you let the valve of a blown-up balloon
go; the static pressure is converted to velocity pressure as the balloon
flies through the air.
It is interesting to note that pitot tubes are a common
protrusion from the fuselage of airplanes as an air-speed measurement device.
To ensure accurate velocity pressure readings, the tip of the pitot tube
must be pointed directly toward the airstream.
When the pitot tube is correctly aligned, the velocity pressure reading will be at its
maximum. Because accurate readings cannot be taken in a turbulent airstream,
the pitot tube should be inserted at least 8-1/2 duct diameters downstream
from elbows; bends or other fittings/obstructions that cause turbulence.
In order to make accurate readings, the pitot tube must be kept free of
dust and debris.
for ASHRAE 62.2
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